Many miles of roadways and many acres of parking lots are paved with asphalt concrete, which is comprised of approximately 94% by weight aggregate materials and approximately 6% by weight asphalt cement. The asphalt cement binds the aggregate materials together, forming a strong but flexible pavement. Although technically a liquid, asphalt cement is extremely viscous, approaching a solid at ambient temperatures. One problem with the asphalt cement component of a roadway is that it cracks when cold and deforms when hot. Pavement life can be extended significantly by adding elastomeric polymers such as styrene-butadiene-styrene (“SBS”) block copolymers or styrene-butadiene rubber (“SBR”) latex to the asphalt concrete composition. It is also known to include reclaimed rubber from recycled tires, ground tire rubber (“GTR”), polyethylene, polypropylene, ethyl-vinyl-acetate (“EVA”), polyvinyl chloride (“PVC”), gilsonite, sulfur cross-linking agents, liquid additives, liquid or solid anti-stripping agents, flux oil, polyolefins or blends of these materials in the composition.
In many circumstances, it is desirable to add the additives such as elastomeric polymers to the asphalt cement prior to mixing the asphalt cement with the aggregates. However, because the use of particulate plastics in asphalt concrete is believed to strengthen the paving material by adding a slightly flexible interlocking aggregate component that bonds with the asphalt cement with a partially chemical molecular bond, it is not desirable to melt the polymers in the mixing process. Polymer additions of this type produce increased shear resistance in the paving material and make it more highly impermeable to water, preventing such water from propagating into the underlying base or subgrade. Consequently, polymers are frequently added to asphalt cement in pellet form.
Asphalt cement is commonly mixed with polymer pellets in a mixer having one or more impellers or mixing blades. Sometimes the polymer pellets are mixed with the asphalt cement in a horizontal mixing tank having impeller assemblies spaced along the length of the tank. In other configurations, polymer pellets are mixed with asphalt cement in a vertical tank having multiple impeller assemblies disposed along the length of a single vertical mixing shaft. When polymer pellets are blended with asphalt cement in conventional mixers, the mixing tanks are so large that the polymer pellets are in contact with the hot asphalt cement for a relatively long time. This extended contact time causes the pellets to partially melt and deform. Thereafter the hot mixture is usually passed through a grinding mill to reduce the asphalt cement coated pellets to smaller particulates. Because of the deformation of the polymers in the mixing tanks, the grinding mills pull and stretch some of the polymer components rather than grinding them. Some of the polymer particulates will then rejoin with other particulates. When this happens, it may be necessary to pass the mixture through the grinding mill multiple times in order to obtain a proper dispersion of the polymer particulates in the asphalt cement.